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Field Sobriety Tests


Most drivers and even many police officers are not aware that there are only three field sobriety tests that the government recognizes. The Standardized Field Sobriety Test (SFST) is a battery of three tests used to detect indicators of impairment and establish probable cause for an arrest for DUI. These tests were developed as a result of research sponsored by the National Highway Traffic Safety Administration (NHTSA) and conducted by the Southern California Research Institute. The three components of the SFST are:

  • Horizontal gaze nystagmus (HGN)

  • Walk-and-turn

  • One-leg stand

These tests are administered systematically and are evaluated according to measured responses of the suspect.


Horizontal Gaze Nystagmus.  An involuntary jerking of the eyeball that occurs naturally as the eyes gaze to the side. Under normal circumstances, nystagmus occurs when the eyes rotate at high peripheral angles. When a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles. An alcohol-impaired person will also often have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a suspect as they follow a slowly moving object, often a pen or small flashlight, horizontally with his or her eyes. The examiner looks for three indicators of impairment in each eye:

  • If the eye cannot follow a moving object smoothly

  • If jerking is distinct when the eye is at maximum deviation

  • If the angle of onset of jerking is within 45 degrees of center


HGN may also indicate consumption of seizure medications, phencyclidine, a variety of inhalants, barbiturates, and other depressants.


Divided Attention Testing. The walk-and-turn test and one-leg stand test are "divided attention" tests that are supposed to be easily performed by most sober people. They require a suspect to listen to and follow instructions while performing simple physical movements. Impaired persons typically have difficulty with tasks requiring their attention to be divided between simple mental and physical exercises.  In the walk-and-turn test, the subject is directed to take nine steps, heel-to-toe, along a straight line. After taking the steps, the suspect must turn on one foot and return in the same manner in the opposite direction. The examiner looks for seven indicators of impairment:

  • Suspect cannot keep balance while listening to the instructions

  • Begins before the instructions are finished

  • Stops while walking to regain balance

  • Does not touch heel-to-toe

  • Uses arms to balance

  • Loses balance while turning

  • Takes an incorrect number of steps.


Testing Accuracy. The NHTSA manual says that if the FST tests are not performed properly, or if conducted without adhering to the training protocols, such actions "compromise" the validity of these evaluations.

The HGN evaluation, when performed correctly on proper subjects, had a 77% "claimed" reliability rating. The walk-and-turn exercise, when conducted properly on a qualified subject on a dry, level surface, was found to be 68% reliable. The one-leg stand exercise, when conducted properly, on a qualified subject on a level, dry surface and under proper instructions and where correctly demonstrated and scored, reportedly yields about 65% reliability. Cumulatively, if all are done correctly, up to 83% correlation to a BAC of .10% or more may be expected.  A knowledgeable DUI lawyer knows that 95% or more of the officers administering these evaluations do them wrong, or conduct them in a manner not approved by the SFST manual, or grade the evaluations improperly, or all of the above. When done incorrectly, these evaluations have zero predicted reliability.


Alternative Testing Methods. Sometimes, an officer will encounter a disabled driver who cannot perform the SFST. In such cases, some other battery of tests such as counting aloud, reciting the alphabet, or finger dexterity tests may be administered.  Tests given by some officers may include picking up coins off the ground, patting or hand clapping, estimating time, counting backwards, reciting the alphabet, touching fingers to the thumb in a pattern set by the officer, or touching index fingers to the tip of the nose while the person's eyes were closed and head tilted back.  Some "non-standardized" tests are so ridiculous and difficult that proof of non-validity is easy with almost any jury or judge. Today, officers who lack NHTSA training invariably cannot cite any studies or scientific research that "validated" their tests, the scoring or their testing methods. Typically, no scoring system is used on tests that do not adhere to NHTSA guidelines. If non-standardized tests are used, the number of errors that are required for a subject to fail is totally subjective, depending entirely upon the officer's opinion. Hence, the untrained officer is usually an easy target for an experienced defense attorney who knows the limitations of these field tests.


Voluntary Nature of the FST. These tests are voluntary, and you are in no way required to take them. In most instances, it is advisable to refuse to take field sobriety tests, due to the fact that most police officers lack the skill and training deliver them 100% standardly. You may fail the test even if you are not impaired by alcohol or drugs.  Nearly any DUI attorney will tell you "NO, do not attempt ANY field tests-EVER." They know that m any studies have concluded that the SFSTs are "designed to fail" even sober drivers.  A motorist's alleged poor performance on field evaluations may provide the probable cause or legal justification necessary for an officer to arrest a person for impaired driving and may become part of the proof used to later convict the person at trial. Therefore, it is of the highest importance that you work with an attorney who knows as much or more about these tests as the police. 


Challenging a Field Sobriety Test.  An attorney can challenge the subjective nature of FSTs, the accuracy of the principles behind the tests, the accuracy of the administration of the tests, the credibility of the officer who "requested" the tests, and challenge all circumstances connected with the evaluations. An attorney representing you may attack the factual and legal issues that may arise regarding the officer's scoring and evaluation of the field tests.  The reason that most credible scientists across America and in other countries are unwilling to categorize field tests-even NHTSA's tests-as being scientific is that too many variables are involved in roadside testing to ever eliminate pure chance and non-controlled circumstances from the equation, for example environmental conditions such as lighting and roadway slope.  Even NHTSA admits that under optimal conditions, e.g. in an air-conditioned and well-lit room , 35% of sober, drug-free subjects get inaccurate results on the one-leg stand test, 32% of sober subjects get flawed results on the walk-and- turn, and 23% of sober subjects are inaccurately said to be "over the legal limit" on the horizontal gaze nystagmus test. In comparison, lie detector tests are more than 90% accurate when conducted by a qualified operator, and unless agreed to by both the defense and prosecution, are still not permitted into evidence by most courts.


Roadside Alcohol Screening Tests.  A portable breath testing device ("PAS") may be used by police officers in determining whether a motorist is under the influence of alcohol. Our state previously banned the use of these voluntary "non-evidential" screening devices for use at trial, but allows it at a pre-trial hearing at which "probable cause" for arrest is involved.  Like other "field tests", these devices are used on the roadside. Police officers do not typically check the devices for calibration on a regular schedule. Furthermore, they routinely ignore the manufacturer's instructions, e.g., failing to observe a 15-minute deprivation period, waiting at least 4-minutes between tests, or clearing the prior test results. Title 17 of the California Administrative Code contains all the information relevant to what constitutes a valid blood or breath test.  Some police agencies try to use these roadside testers as evidence. This is accomplished when a small printer is attached to the breath test apparatus. Unless your jurisdiction uses such a device as an official state-mandated breath test, no person should ever submit to these devices and risk a false positive result and almost certain arrest. Politely decline to give this voluntary sample, although be aware that refusal of an official breath or blood test later can be considered a refusal, triggering a license revocation.

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